Update layout/head.ejs, improve og image supoort (#440)

includes/helpers/page.js

@@ -8,6 +8,7 @@
 *     <%- meta(post) %>
 *     <%- has_thumbnail(post) %>
 *     <%- get_thumbnail(post) %>
+*     <%- get_og_image(post) %>
 */
 module.exports = function (hexo) {
     function trim(str) {
@@ -93,4 +94,29 @@ module.exports = function (hexo) {
         const hasThumbnail = hexo.extend.helper.get('has_thumbnail').bind(this)(post);
         return this.url_for(hasThumbnail ? post.thumbnail : 'images/thumbnail.svg');
     });
-}
+
+    hexo.extend.helper.register('has_og_image', function (post) {
+        return post.hasOwnProperty('og_image');
+    });
+
+    hexo.extend.helper.register('get_og_image', function (post) {
+        const getConfig = hexo.extend.helper.get('get_config').bind(this);
+        const hasConfig = hexo.extend.helper.get('has_config').bind(this);
+
+        const hasOGImage = hexo.extend.helper.get('has_og_image').bind(this)(post);
+        const hasThumbnail = hexo.extend.helper.get('has_thumbnail').bind(this)(post);
+
+        const getThumbnail = hexo.extend.helper.get('get_thumbnail').bind(this);
+
+        let og_image
+
+        if(hasOGImage)
+            og_image = post.og_image
+        else if (hasThumbnail)
+            og_image = getThumbnail(post);
+        else
+            og_image = getConfig('article.og_image', '/images/og_image.png');
+
+        return this.url_for(og_image);
+    });
+}

layout/common/head.ejs

@@ -4,17 +4,14 @@
 <%- meta(page) %>
 
 <% if (has_config('open_graph')) { %>
-    <% const og_options = {
+    <%- open_graph({
         twitter_id: get_config('open_graph.twitter_id'),
         twitter_site: get_config('open_graph.twitter_site'),
         google_plus: get_config('open_graph.google_plus'),
         fb_admins: get_config('open_graph.fb_admins'),
-        fb_app_id: get_config('open_graph.fb_app_id')
-    };
-    if (has_thumbnail(page)) {
-        og_options['image'] = get_thumbnail(page);
-    } %>
-    <%- open_graph(og_options) %>
+        fb_app_id: get_config('open_graph.fb_app_id'),
+        image: get_og_image(page)
+    }) %>
 <% } %>
 
 <% if (has_config('canonical_url')) { %>